1. Field of the Invention
The present invention relates generally to the field of optical waveguide fibers, and more particularly to methods of making low polarization-mode dispersion optical waveguide fibers.
2. Technical Background
A significant goal of the telecommunications industry is to transmit greater amounts of information, over longer distances, in shorter periods of time. Typically, as the number of systems users and frequency of system use increases, demand for system resources increases as well. One way of meeting this demand is by increasing the bandwidth of the medium used to carry the information. In optical telecommunication systems, the demand for optical waveguide fibers having increased bandwidth is particularly high.
An optical fiber is typically formed from a silica-based preform. The preform may be manufactured, at least in part, by depositing silica soot or silica plasma onto a receptor surface and building up layers of silica, wherein the soot stream attaches to the receptor surface either in soot form, as in an OVD process, or in fused silica form, as in an MCVD process. The receptor surface is typically a mandrel (e.g. the outer surface of a ceramic bait rod) or a tube of fused silica (e.g. the inside surface of the tube in an MCVD process). The receptor surface is typically rotatable or rotating during deposition. Prior to consolidation, the mandrel or bait rod is separated from the preform, leaving a center hole. After consolidation, additional layers of silica soot, e.g. overclad or additional core material, may be applied and added to the preform. Preforms which at least partially contain soot are eventually consolidated into fused silica before being drawn into an optical fiber. Some preforms are made by a rod in tube process wherein a fused silica rod is inserted into the central opening of a fused silica core.
In the manufacturing of an optical fiber, a variety of methods can be used to deposit the various soot layers of a preform.
In the outside vapor deposition (“OVD”) process, an unconsolidated soot preform such as a soot core blank, is formed by depositing silica and germanium containing precursor constituents in the presence of oxygen onto a substrate, such as a mandrel, or a target rod, typically a ceramic bait rod. As the bait rod is rotated, the precursor constituents are delivered to the flame burner to produce soot, and that soot is then deposited onto the bait rod. The soot may be a combination of silica and silica-doped soot. Once sufficient soot is deposited, the bait rod is removed, and the resultant soot core blank can be consolidated into a fused silica preform such as a core rod preform or core cane preform or cane preform or glass core blank. The soot core blank is typically consolidated by hanging the soot core blank in a consolidation furnace and heating the soot core blank to a temperature and for a time sufficient to consolidate the soot core blank into a glass. Preferably, prior to the consolidating step, the soot core blank is chemically dried, for example, by exposing the soot core blank to chlorine gas at an elevated temperature. The result is a generally cylindrical glass core blank or glass cane preform having an axial hole along its centerline, or centerline hole, or centerline aperture. That is, the generally cylindrical consolidated glass tube has a centerline aperture. Typically, the glass core blank or glass cane preform has a length of about 0.5 m to 1.0 m, with an inside diameter of about 0.5 to about 3.0 cm, and an outside diameter of about 3 to 8 cm. Although these dimensions vary according to process and product requirements, various sizes and even shapes of glass core blank or glass cane preform can benefit from the present invention as set forth hereinbelow.
The glass core blank or glass cane preform is then typically “redrawn”, i.e. drawn in to a reduced diameter preform, e.g., by positioning the glass core blank in a furnace, heating the core blank to a temperature of approximately 2000° C., and then redrawing or pulling or stretching the core blank into a smaller diameter core cane. The thermal energy softens the glassy blank or preform which, in tandem with pulling on the preform, results in a necking down of the preform, i.e. necking of both the outer diameter and the inner diameter.
During the redraw operation, the centerline aperture of the core blank is typically collapsed by applying considerable vacuum (e.g., a pressure of less than 25 kPa or 0.25 atm) along the centerline aperture. These vacuum forces ensure complete closure of the glass core blank along the centerline. Typically, drawing or pulling on the preform without the assistance of vacuum is insufficient to close or collapse the hole.
After the redraw step, the resulting core cane is then typically clad with one or more additional core soot layers and/or overclad with a layer of cladding by depositing a cladding soot thereon, e.g. via an OVD deposition process. Once covered with sufficient cladding soot, the resultant soot overclad core cane is chemically dried and consolidated to form an optical fiber preform or optical fiber blank. While different processes (e.g. MCVD and others) may employ somewhat different processes to form components employed in the manufacture of preforms, many of these processes (e.g. MCVD) commonly result in a cylindrical tube or other intermediate glass object or preform having a hole therein, which is closed prior to drawing optical fiber therefrom. These manufacturing processes typically involve utilizing a vacuum at some point during the manufacturing process to close the centerline aperture without changing the outer diameter significantly.
The use of a relatively strong vacuum to close the centerline and other apertures in a glass core blank or other optical fiber preforms typically presents difficulties. Such vacuum forces can result in a non-symmetrical centerline profile of the cane, as shown, for example, in FIG. 1. The application of strong vacuum to the centerline aperture region can result in a non-circular collapse of the hole. FIG. 1 illustrates a cross section of core cane, indicated generally at 10, which includes a center point 12 surrounded by layers of glass 14. In FIG. 1, these glass layers 14 have an irregular, asymmetric shape, as a result of the application of strong vacuum forces which result in the full collapse of the centerline aperture. Only at locations farther from the center point 12 do the layers of glass 16 begin to form more symmetrical and concentric circles or rings about the center point 12. The same non-symmetrical layers of glass present in the core cane will be present when that cane is eventually drawn into an optical fiber. Views of the centerline profile taken at different locations along the length of the core cane (or the optical fiber resulting therefrom) would also show core asymmetry. Further, the geometrical properties of the core cane and resultant optical fiber may change along the length thereof. More specifically, the specific asymmetrical shape at one location along the optical fiber might differ from the shape at another location along the optical fiber.
Asymmetric core geometry is believed to be a key cause of polarization mode dispersion (PMD), a form of dispersion which results when one component of light travels faster than another, orthogonal component. The occurrence of PMD which is present to any significant degree, especially in single mode fibers, is a severe detriment because PMD limits the data transmission rate of fiber-based telecommunications systems. More specifically, single mode fibers and multimode fibers typically both have an outside diameter of generally about 125 microns. However, single mode fibers have a relatively small core diameter, e.g., about 8 microns. Because of this dimensional relationship, single mode fibers are extremely sensitive to polarization mode dispersion brought on by non-symmetric hole closure caused during fiber manufacture. Consequently, reduced PMD is a significant goal in fiber manufacture, especially in single mode fibers. In contrast to the small core size of single mode fibers, the core region of a multimode fiber typically has a diameter of 62.5 microns or 50 microns. PMD is also deleterious in multimode fibers. In multimode fibers, non-symmetric hole closure has resulted in the inability to tune refractive index profiles on the innermost portion of the fiber adjacent the centerline. As a result, lasers used to launch light into such fibers are often offset some distance from the centerline of the multimode fiber to avoid this region of non-symmetric hole closure. Thus, both single mode and multimode fibers could benefit from lowered PMD.
One method used to reduce PMD is spinning of the optical fiber during the fiber draw operation, wherein the fiber is mechanically twisted along its centerline axis while being drawn from the molten root of the optical fiber preform or blank. This twisting enables orthogonal components of light to couple to each other, thus averaging their dispersion and lowering PMD. Although spinning can mitigate the effects of non-symmetric hole closure, spinning is a fairly complicated process which can detract from an optical fiber and/or the manufacture thereof. For example, spinning can impede the speed at which fiber is drawn, cause coating geometry perturbations, reduce the strength of the optical fiber, and so forth.
Additionally, asymmetric core geometry can cause variations in core diameter along the length of the fiber core so that light transmitted through the fiber propagates through or “sees” a different core cross-sectional area at different points along the length of the optical fiber. In addition, an asymmetric centerline profile can reduce the bandwidth of laser launched multimode fiber.
The use of strong vacuum forces to close the centerline aperture may also result in voids being formed along the centerline which can further impair the transmissive properties of the optical fiber.
As used herein, the term “preform” refers to any silica-based body used in the manufacture of optical waveguide fiber, whether containing silica soot or not, including but not limited to preforms also known as unconsolidated soot preforms, soot core preforms, soot core blanks, fused silica preforms, core rod preforms, core cane preforms, core blanks, glass core blanks, glass cane preforms, glassy preform, consolidated preform, and/or optical fiber preforms.